1. Field of the Invention
This invention relates to a method for fabricating a flash memory cell, and more particularly, to a method for forming a stacked gate.
2. Description of Related Art
Recently, high-density flash memories have been receiving much attention for application in many fields. One of the most important factors is the low cost of the reduced size of each flash memory cell. However, it is very hard to shrink the cell size in the fabrication of conventional flash memory cells, because a local oxidation (LOCOS) isolation technique is used. The isolation structure formed by the technique limits a reduction of the flash memory.
Another isolation technique called shallow trench isolation (STI) has been disclosed. The fabrication of a flash memory cell comprises the implementation of the STI technique to reduce the cell size. The small-sized cell benefits the integration of the flash memory cell on a wafer. However, the coupling ratio of the cell decreases as the size of the cell becomes small. The decrease is because the overlapping area between the floating gate and the control gate in the cell is reduced.
When the coupling ratio is low, a higher voltage must be supplied in order to operate the memory programming and erasing actions. A high operating voltage makes any dimensional reduction of the flash memory cell very difficult. On the other hand, for a flash memory cell having a high coupling ratio, the electric field necessary to initiate an F-N tunneling is high, thereby slowing the tunneling speed of electrons from the floating gate to the source/drain region. The effect slows down the speed of a read or a write operation in the flash memory.